In a typical cellular system, call establishment begins either by a base station transmitting a paging message to a mobile station on a paging channel and, then the mobile station transmitting a paging response message to the base station on an access channel, or, by a mobile station accessing the system on an access channel by transmitting an origination message to a base station. In either of these call establishment cases, the mobile station must obtain access to the system on an access channel, and setup information unique to the particular call establishment must be exchanged between the mobile station and base station over the access channel or other channels of the system air interface. In certain systems, the paging response message and origination message may carry a large portion of this setup information. The setup information unique to the particular call establishment could include called number data, mobile station identification and capability related data, authentication information, etc. After receiving this information, the system must then use the information to setup the different layers of communication necessary in the system to implement the call.
If a connection in an ongoing call in a cellular system is broken while the call is in progress, the call must be re-established in order for the call to be completed. For example, an interruption in communications on the air interface may result in a voice channel being interrupted long enough for a call to be dropped. The call must then be re-established.
Major cellular system types include those operating according to the Global Services for Mobile(GSM)Standard, the TIA/EIA/IS-95 Mobile Station-Base Station compatibility Standard for Dual Mode Wide Band Spread Spectrum Cellular System, the TIA/EIA/IS-136 Mobile Station-Base Station Compatibility Standard, and the TIA/EIA 553 Analog Standard (AMPS/TACS). Other major cellular systems include those operating according to the IS-95 based ANSI-J-STD-008 1.8-2.0 Ghz standard or, those operating according to the GSM based PCS1900 (1900 Mhz frequency range) standard. Systems operating according to the IS-95, IS-136 and AMPS/TACS systems standards do not provide a mechanism for re-establishing a call other than for one of the parties involved in the call to manually reinitiate the call. When this happens the mobile station and base station must re-execute the call establishment procedure, using identical messaging containing the same information as was exchanged when the call was first established.
Systems based on the GSM standard may support a mobile station triggered call re-establishment procedure that uses a call re-establishment message that includes only the subscriber identity of the mobile station and the mobile station's classmark. The procedure involves sending an access request message from the mobile to the system that indicates a re-establishment is requested, accepting a channel assignment from the system and, transmitting the re-establishment request message on the assigned channel. After receiving the call re-establishment message, a GSM system must reconstruct the connection using only the subscriber identity and classmark from information saved in the network.
GSM call re-establishment depends upon the mobile station triggering the re-establishment procedure. The system has a role in triggering re-establishment only so far as it indicates to the mobile station during call setup that it supports the call re-establishment procedure. If the system supports call re-establishment, the GSM specification makes it mandatory for the mobile station to initiate call re-establishment when a connection is lost during a call.
While mobile station triggered call re-establishment procedures similar to the GSM call re-establishment procedure could have application for re-establishment of conventional calls in other digital cellular systems presently in use, additions to the specifications for these other systems, that expand the types of services available, create a need for a more flexible call re-establishment procedure.
Currently, for example, the major cellular systems standards bodies are implementing packet data services into their digital cellular specifications. A packet data service specification has been finalized for GSM, and packet data service specifications compatible with the IS-95 and IS-136 standards are being prepared. The TIA/EIA IS-657 packet data services specification and TIA/EIA TSB-74 Telecommunications Systems Bulletin define a connection based packet data service for IS-95 based networks. In connection based packet data service, a physical circuit connection for the call is maintained by the cellular system during the duration of the packet data call. The IS-657/TSB-74 system allows transmission of packet data at rates up to 14.4 kbps. It has also been proposed to implement high speed packet data (HSPD), at rates of up to 78.8 kbps, into a standard based on IS-657 and TSB-74.
In an application such as IS-657/TSB-74 connection based packet switching service, the GSM-type call re-establishment procedure would not provide the most efficient method of re-establishing a lost connection. IS-657/TSB-74 connection based packet data uses a timer feature in each of the base station/mobile switching center(MSC) and mobile station. A packet inactivity timer in each of the base station/mobile switching center and mobile station are reset whenever packet data frame is sent or received. If the packet inactivity timer expires before more data is sent or received, in either the mobile station or base station/MSC, the mobile station or base station/MSC in which the packet inactivity timer expired disconnects the packet switching service option by sending a release order message. The release order message causes the connection to be released by both the mobile station and base station/MSC. If either the mobile station or base station/MSC has remaining packet data for the call to send after the connection is released, the mobile station or base station/MSC needing to send the packet data must re-establish the call using the identical procedure used to initially establish the call. The GSM call re-establishment procedure would not be useful in an IS-657/TSB74 re-establishment situation because a packet data call may involve a unidirectional exchange of packet data, and initiation of re-establishment of a packet data call may be required from either the mobile station or base Station/MSC. Re-establishment in IS-657/TSB-74 includes using an origination message identical to that initially used to establish the call, and system access procedures, as used for the initial call origination process, are employed. The system access procedure involves transmitting a succession of successively higher powered access probes, that include the origination message, until an acknowledgment is received from the system or a time limit is reached and the access attempt is aborted. In an IS-95 based packet data system the access attempt could take up to 3120 ms, depending upon the number of access probes that must be transmitted before acknowledgment is received from the system by the mobile station. In packet data applications, especially ones involving high speed data, call re-establishment times having this duration may have a negative effect on system performance.
It would provide an advantage, then, to have a method and apparatus for packet data call re-establishment that allows faster re-establishment of a dropped call than can be accomplished by repeating the initial establishment protocol used to first establish the call. Additionally, since packet data may be transmitted from either end of a link independently of the other, it would provide an advantage if the method and apparatus for re-establishing the call allowed either one of a pair of devices in a packet call to initiate call re-establishment.